1. Field of the Invention
The present invention relates to a method of preventing polymer scale deposition in polymerization of a monomer having an ethylenic double bond, and a polymer scale preventive agent used for the method.
2. Description of the Prior Art
In processes of preparing polymers by polymerizing a monomer in a polymerization vessel, the problem that polymer deposits on the inner wall surface and so forth in the form of scale, is known. The deposition of the polymer scale on the inner wall results in disadvantages that the yield of the polymer and the cooling capacity of the polymerization vessel are lowered; that the polymer scale may peel and mix into manufactured polymer, thereby impairing the quality of the manufactured polymer; and that removal of such polymer scale is laborious and hence time-consuming. Further, since the polymer scale contains unreacted monomer and operators may be exposed thereto, which may cause physical disorders in the operators.
Heretofore, as methods for preventing polymer scale deposition on the inner wall surface and so forth of a polymerization vessel, methods in which substances exemplified below are coated on the inner wall surface, etc. as a scale preventive agent, have been proposed. For example, particular polar organic compounds (Japanese Patent Publication (KOKOKU) No. 45-30343(1970), a dye or pigment (Japanese Patent Publication (KOKOKU) No. 45-30835(1970), an aromatic amine compound (Japanese Pre-examination Patent Publication (KOKAI) No. 51-50887(1976)) and a reaction product of a phenolic compound and an aromatic aldehyde (Japanese Pre-examination Patent Publication (KOKAI) No. 55-54317(1980)).
These methods can fairly effectively prevent the deposition of polymer scale in the polymerization of a vinyl halide monomer such as vinyl chloride or a monomer mixture mainly comprised of such a monomer.
In the meantime, vinyl chloride polymers obtained by polymerization are required to have a good whiteness; for example, they are required to have an L value according to the Hunter's color difference equation, which is described below, of 70 or more.
The prior art polymer scale preventive agents generally include a great number of colored substances as exemplified typically by the dyes and pigments described in Japanese Patent Publication (KOKOKU) No. 45-30835(1970) or 52-24953(1977), the aromatic amine compounds described in Japanese Pre-examination Patent Publication (KOKAI) No. 51-50887(1976), and the reaction products of a phenolic compound and an aromatic aldehyde described in Japanese Pre-examination Patent Publication (KOKAI) No. 55-54317(1980). Although these polymer scale preventive agents can fairly effectively prevent polymer scale deposition in the polymerization of a vinyl halide monomer such as vinyl chloride or a monomer mixture mainly comprising a vinyl halide as mentioned above, where the coating comprising such a colored polymer scale preventive agent is dissolved, causing contamination of the produced polymer, a problem may arise that the produced polymer become colored and its quality is seriously lowered. That is, according to measurement of the lightness L in the Hunter's equation described in JIS Z 8730 (1980), the L may be measured to be 65 or less, and coloration is thereby confirmed. Presumably, the coloration is caused by incorporation of components of the coating which has dissolved or peeled into the polymerization system.
Moreover, terrible substance such as aniline, nitrobenzene, formaldehyde, etc. among the polar compounds described in the above-mentioned Japanese Patent Publication (KOKOKU) No. 45-30343(1970), and dyes containing a heavy metal such as chromium or lead among the dyes described in Japanese Patent Publication (KOKOKU) No. 45-30835(1970) are poisonous. The dyes described in Japanese Patent Publication (KOKOKU) Nos. 45-30835(1970) and 52-24953(1977) include some dyes that involve concern about carcinogenesis. Therefore, use of these substances may cause problems in safety of operators.